The long-term objective of this application is to characterize what appears to be a new cellular mechanism for regulating the activity of protein phosphatase 2A (PP2A), an enzyme which plays a critical role in modulating cell growth regulatory pathways by dephosphorylating key proteins in these pathways. Specifically, we propose that a protein called HSF2 regulates PP2A activity by binding to the PR65/A subunit of PP2A and preventing its association with the catalytic subunit by directly competing with catalytic subunit for its binding site in PR65. The biological relevance of HSF2's ability to block interaction between PR65 and catalytic subunit is demonstrated by results showing that mutations in PR65 that prevent binding of catalytic subunit are associated with human lung and colon cancers. HSF2 is also known to be a transcription factor which regulates expression of heat shock protein (hsp) genes, and thus we hypothesize that HSF2 regulation of PP2A could serve as a mechanism for cross-talk between control of hsp expression and cell growth regulatory pathways. To elucidate the function of HSF2 as a novel regulator of PP2A we propose to 1) identify the sequences in HSF2 and PR65 important for this interaction, 2) characterize cellular mechanisms which regulate the HSF2:PR65 interaction, 3) determine the function of HSF2 in regulating cellular PP2A activity, and 4) identify other potential members of this new class of PP2A regulatory protein using yeast two-hybrid interaction screens (we have already identified one new candidate using this meth6d, a growth regulatory protein called mel-18). These studies will advance knowledge by elucidating a new mechanism of PP2A regulation as well as a new mechanism for cross-talk between two important signaling pathways in cells, hsp regulation and control of cell growth/differentiation. This knowledge could aid understanding of both normal cell growth regulation as well as aberrant proliferation control in cancer cells.